1. Field of the Invention
The present invention relates to a high-pressure metal-vapor discharge lamp lighting apparatus which lights a high-pressure metal-vapor discharge lamp so as to raise a quantity of light of the lamp early, and a headlight device for a vehicle using the lighting apparatus.
2. Description of the Related Art
A method of starting or igniting a high-pressure gas discharge lamp for use in headlights of an automobile is described, for example, in Jpn. Pat. No. 3249145.
In this method, an AC (alternating-current) voltage is formed from a DC (direct-current) voltage of a battery, the AC voltage is supplied to an igniting apparatus in order to ignite the high-pressure gas discharge lamp, a lamp voltage and/or a lamp current is measured, and actual values of the lamp voltage and/or the lamp current are compared with target values. An auxiliary power is supplied in addition to an operation power of a high-pressure metal-vapor discharge lamp by a change of the AC voltage over a predetermined period after the igniting of the high-pressure gas discharge lamp, and the AC voltage is restricted by an allowable maximum lamp current on one hand, and is lowered after elapse of a predetermined period on the other hand. Furthermore, state values indicating values “lamp on” and “lamp off” are formed, and the auxiliary power (ZL) to be formed after the igniting is increased with an increase of a cutoff (off) period of time and with a decrease of a period of the previous lighting (on) time. Moreover, a period (T) in which the auxiliary power (ZL) is effective increases with the increase of the cutoff period of time and with the decrease of the period of the previous lighting period of time of the high-pressure gas discharge lamp.
On the other hand, at present, for the high-pressure discharge lamp for use in a lighting apparatus for an automobile, mainly mercury for forming the lamp voltage which is a main luminescent material for obtaining a white light, sodium Na and scandium Sc which are halides, and xenon which functions as a starting gas and buffer gas to compensate for emission immediately after the lighting are respectively sealed into a luminous tube. Therefore, the high-pressure discharge lamp constitutes a metal halide lamp which is one type of high-pressure metal-vapor discharge lamp. It is to be noted that the high-pressure discharge lamp whose main discharge medium is a metal-vapor is classified as the high-pressure metal-vapor discharge lamp, and the high-pressure discharge lamp whose main discharge medium is gas is classified as the high-pressure gas discharge lamp.
On the other hand, mercury is a material large in environmental load, and the invention for removing this material is described, for example, in U.S. Pat. No. 6,353,289. That is, in the invention described in the U.S. Pat. No. 6,353,289, halide of a metal which is comparatively high in steam pressure and which does not easily emit light in a comparatively visible range is added as a second halide to a first halide of a luminous metal, and at least xenon having an atmospheric pressure or more is sealed into an airtight container having fire resistance and transmittance. A light in a chromaticity range of the white light is emitted immediately after the lighting by a combination of the first and second halides and xenon in the airtight container in the metal halide lamp, that is, the high-pressure metal-vapor discharge lamp. This conventional invention has realized an automobile headlight including the high-pressure metal-vapor discharge lamp (hereinafter referred to as “mercury-free lamp” for convenience in the specification) in which mercury is not substantially sealed.
When a quantity of light indicating a predetermined value or more is required in a predetermined time from the lighting is required as in the high-pressure metal-vapor discharge lamp for use in the headlight for the automobile, a lamp power larger than a rated lamp power is supplied at a starting time at a temperature of so-called room temperature to reduce a rising time of the quantity of light. This technique is known, for example, by the Jpn. Pat. No. 3249145. When this technique is applied to the conventional high-pressure metal-vapor discharge lamp (hereinafter referred to as the “mercury-containing lamp” for the convenience) using mercury to form the lamp voltage, first mercury having a low boiling point evaporates to emit light. With a rise of the temperature in the tube, the halide of the luminous metal evaporates to contribute to desired emission. Therefore, when the quantity of light obtained by the mercury emission is not less than the predetermined value first in the starting, the initial emission contribution of the luminous metal can be provided over a comparatively long time. For example, even with the lighting with a large power of about 70 W immediately after the starting, a steam pressure of mercury is saturated in about one to two seconds. Thereafter, even with the lighting at a lamp power smaller than 70 W, a quantity of light equal to that at a stable time can be anticipated, and several tens of seconds can be spent from when the evaporation of the halide of the luminous metal starts until the emission by the luminous metal becomes dominant. Therefore, even with fluctuations in a shape of the high-pressure discharge lamp, a charged amount of the halide of the luminous metal, a distributed state of the halide sticking to the inside of the tube and the like, comparatively stable rising characteristics of corresponding quantity of light can be obtained. However, there has been a demand for more stable rising characteristics of the quantity of light.
On the other hand, for the mercury-free lamp, since the emission of mercury cannot be expected in a starting initial stage, rare gases such as xenon first emit light, and subsequently the emission of the luminescent metal charged as the halide starts.
Additionally, in the mercury-free lamp, a very high temperature is required for the evaporation of the halide of the luminous metal as compared with mercury. Therefore, when the quantity of light equal to that of the mercury-containing lamp is to be obtained in an equal time, the lighting at the lamp power higher than a rated lamp power by 2.5 times or more has to be projected for a time much longer than that in the mercury-containing lamp. Additionally, when the high lamp power is projected, a phenomenon occurs in which the charged metal halide rapidly evaporates and the quantity of light is twice or three times that stabilized under the rated lamp power.
When the rapid evaporation of the metal halide occurs without any control in the mercury-free lamp as described above, the emission occurs with a quantity of light much larger than a rated quantity of light. As a result, for the headlight device for the automobile, a driver of an oncoming car is intensely dazzled, and this is remarkably dangerous.
Moreover, the mercury-free lamp does not have the emission by mercury. Therefore, even when the projected lamp power is constant, the rising characteristics of the quantity of light in the high-pressure metal-vapor discharge lamp remarkably fluctuate by fluctuations at a manufacturing time. That is, the fluctuations of the rising characteristics of the quantity of light cannot be predicted in consideration of a manufacturing tolerance of the high-pressure metal-vapor discharge lamp. That is, a processing precision of the airtight container, a charged amount or constituting ratio of the halide of the discharge medium, rare gas charged pressure or the like cannot strictly be managed. Moreover, since the high-pressure discharge lamp is constituted of elements that easily fluctuate the rising characteristics of the quantity of light, the rising characteristics of the quantity of light easily fluctuate in a broad range. The fluctuations of the rising characteristics of the quantity of light basically similarly occur both in the mercury-containing lamp and the mercury-free lamp. However, the mercury-free lamp requires a large power at the starting time as compared with the mercury-containing lamp as described above. Additionally, since the steam pressure of the second halide is higher than that of mercury, the lamp characteristics are easily influenced by the fluctuations.
Moreover, a delay is generated in a time required until the quantity of light is stabilized at a set value because of the fluctuations of the rising characteristics of the quantity of light in the high-pressure metal-vapor discharge lamp, and the quantity of light does not successively or smoothly rise. For example, when the high-pressure discharge lamp is lit, the quantity of light temporarily lowers below a rated value, or largely exceeds the rated value in some case.
FIGS. 9A, 9B show changes of a projected lamp power and quantity of light with respect to a lighting time in two conventional different high-pressure metal-vapor discharge lamps, FIG. 9A is a graph of a lamp which is slow in the rising of the quantity of light, and FIG. 9B is a graph of a lamp in which over-emission occurs. It is to be noted that the graph having the ordinate denoted with W shows a change of the projected lamp power with respect to the elapse of time (t), and the graph including the ordinate denoted with lx shows illuminance. Therefore, the change of the quantity of light with respect to the elapse of time (t).
That is, in FIGS. 9A and 9B, the rising characteristics of the quantity of light of the mercury-free lamp are slow. On the other hand, the lamp power drops excessively early. Therefore, the quantity of light once drops. Therefore, extra time is required until the quantity of light is restored again. Eventually, the time lengthens until the quantity of light rises and is stabilized. On the other hand, FIG. 9B shows a case where the rising characteristics of the quantity of light of the mercury-free lamp are early. Even when the lamp power starts dropping, the quantity of light increases excessively early, the reduction of the lamp power does not follow, and the over-emission occurs. It is to be noted that in FIGS. 9A and 9B, the reduction time of the lamp power from the start of the reduction of the lamp power until the rated lamp power is reached is equal at t1.
Moreover, with the high-pressure metal-vapor discharge lamp used in the automobile headlight device, as shown in FIG. 9A, although the headlight device is lit, the quantity of light drops. In this case, visibility drops, and this results in a dangerous state. When the over-emission occurs even temporarily, the oncoming car is intensely dazzled, and this is dangerous.
On the other hand, the rising characteristics of the quantity of light have a dynamic or transient phenomenon, different from the characteristics at a stable time, and therefore the characteristics are not easily managed at the manufacturing time of the high-pressure metal-vapor discharge lamp. Therefore, when the fluctuations of the rising characteristics of the quantity of light are reduced from an manufacturing aspect of the high-pressure metal-vapor discharge lamp, very much trouble and time are required for strictly managing the shape of the airtight container, managing the charged amount of the halide of the discharge medium for each layer, or subtly adjusting the charged gas pressure.
To solve the above-described problem, in accordance with the starting method described in the Jpn. Pat. No. 3249145, in the mercury-free lamp, an average time for which the rapid evaporation of the metal halide occurs is assumed, and the reduction of the projected lamp power is started. However, there are the influences of the rising characteristics of the quantity of light of the high-pressure metal-vapor discharge lamp. That is, in the invention of the Jpn. Pat. No. 3249145, the fluctuations of the rising characteristics of the quantity of light of the high-pressure metal-vapor discharge lamp are not considered. Therefore, the invention described in the Jpn. Pat. No. 3249145 cannot be a solution of the problem caused by the fluctuations of the lamp characteristics of the high-pressure metal-vapor discharge lamp.
On the other hand, the present inventor has developed the invention to solve the above-described problem in the mercury-free lamp. The invention has been applied as Jpn. Pat. Appln. No. 2002-54080 (hereinafter referred to as the “prior application” for the convenience). The invention of the prior application comprises control circuit for controlling lighting circuit to reduce the lamp power to such an extent that a light output is not remarkably large as compared with that at the stable time and does not rapidly increase and for further successively attenuating the lamp power so that the lamp power settles at the rated lamp power, when the lamp power twice the rated lamp power is supplied from the lighting circuit to start the high-pressure discharge lamp and thereafter at least the metal halide charged in the high-pressure discharge lamp rapidly evaporates. The invention of the prior application is solving circuit effective to some degree against the problem caused by the fluctuations of the lamp characteristics of the mercury-free lamp.
However, when the solving circuit effective against the problems caused by the fluctuations of the rising characteristics of the quantity of light of the high-pressure metal-vapor discharge lamp is obtained with a simpler constitution and/or further inexpensively, practical values are high.